I bought mine years ago. My family bought some in 2012. They're still going strong. I want to say they were about $10-12? I'm not sure what they are now, but I'd imagine anywhere from $10-15. I've gotten several trips' worth of use out of them, and use outside of the parks as well at concerts, festivals, and other large outdoor events.
I prefer the heavier vinyl (or vinyl-like) material of the Disney ones. It's thicker, and doesn't cling to my body like the thin bag ones. I find all the dollar-store ones are more or less garbage bags. Not even garbage bags; they're ultra-thin can liners like they use in schools and offices. I've also found that I tend to tear them before they're even on my body.
Once I'm done with them, I roll them up to a small round package that's about the size of a fast-food burrito, and stuff it into a Ziploc bag. I hang it up to dry fully at the hotel each night I've used it. I'd rather reuse a Disney one indefinitely than spend $1-2 each trip or event I need one to buy a cheap disposable. It adds up quickly.